Typography is the art and technique of arranging type, type design, and modifying type glyphs. The arrangement of type involves the selection of typefaces, point size, line length, leading (line spacing), adjusting the spaces between groups of letters (tracking), adjusting the space between pairs of letters (kerning), etc. The legibility of a document is affected by the arrangement of these elements.
The task of arranging type was traditionally performed by professionals, such as editors, typesetters and graphic designers. However, computer authoring tools such as text editors have opened up typographic modifications to people who are not as skilled as professionals. Consequently, a lay person using a computer authoring tool might produce a document that is illegible or at least suboptimal with respect to legibility.
An illegible document might not be immediately apparent to a lay person who authors that document, and the author might not have access to a professional who can evaluate the document's legibility. Even if the author recognized that the document was not legible, the author might not know how to improve the document's legibility.
It is desirable to avoid creating illegible documents.